Friday, July 6, 2012

Advances in Neuronal Destruction

Destroying neurons is not difficult. Destroying specific neurons, but leaving others intact is another story. Ablating specific neurons usually involves fancy genetic trickery, but it can also be accomplished with fancy mechanical lasers!

A new study published in PNAS (Hayes et al., 2012) uses the cells own rhythm generating properties to target the neurons for destruction.

Specifically, Hayes et al. is investigating the breathing neurons. These neurons are in the Pre-Botzinger Complex (preBotC) of the Medulla and they control the inhalation phase of breathing. They work together as a complex to generate rhythms even in a brain slice.

Using a calcium-sensitive dye, Hayes et al. could tell which neurons were participating in the rhythm generation. The breathing neurons show specific calcium patterns, increasing and decreasing with a frequency of 0.15-0.5Hz.

The breathing neurons are located and the specific spatial coordinates of each neuron is saved. A mechanically controlled laser can then automatically target each specific neuron for destruction (red dots in figure below).

Hayes et al., 2012 Figure 1

Because silencing the neurons (NK1R-containing) in the preBotC completely stops breathing, they wanted to see how many neurons could be destroyed before the rhythm stopped. And they wanted to see how it stopped. Is there some magic number of cells that are needed to maintain the rhythmic output? or does the rhythm slowly decrease in amplitude?

So measuring the XII nerve for output, they began randomly destroying the rhythmic cells one by one. They found that destroying these neurons one by one caused a decrease in amplitude and frequency of the XII nerve output and eventually stopped it entirely. It took about 120 neurons to completely stop the rhythm, but the weird thing is that even after destroying 120 neurons, the rhythm continued for about half an hour.

The mechanisms underlying this delay are not completely clear, but the authors attribute it to the slow effects of a decrease in mGluR stimulation.

This new technique is pretty exciting because it allows the sequential deletion of specific cells. Even the study erasing memories cell by cell didn't actually delete the cells one at a time.

This technique is especially interesting for investigating the way that a collection of individual cells create emergent network properties. Now questions like 'how many cells are needed to form or maintain a functional network?' and 'which cells are necessary for the network's function?' can be answered.